The present invention relates to a power detector for detecting the magnitude of a radio frequency (RF) signal.
Power detectors are used, for example, in control circuits at the output of RF signal power amplifiers. Early power amplifiers had only one output power level, that was tuned to the desired level during the production. However, it is now common to reuse radio channels in some RF communications systems, for example, in cellular radio telephone systems. To avoid interference between the two RF signal power amplifiers using the same channel, the two operate at different power levels, and, therefore it is desirable to be able to select different power levels from a number of available power levels. Power detectors are provided in the control circuitry for the RF signal power amplifiers used in such systems. One such detector comprises a rectifier diode the voltage from the RF signal power amplifier being coupled to the anode at the diode, and the cathode providing a rectified voltage proportional to the coupled voltage. The voltage from the detector is coupled to an amplifier and a comparator controlling a current amplifier, from where the current is supplied to the end stage amplifiers. Thus amplifiers and a circuit generating and regulating the current are needed in addition to the power detector.
In order for the power detector to operate at low power levels the rectifier diode must be biased in some way, because the diode has a certain threshold voltage. This threshold voltage is also temperature dependant, so that the biasing must also take into account changes in the threshold voltage caused by temperature.
In order to compensate for changes in the threshold voltage caused by temperature variations it is known to use a second diode, which is forward biased and from which a biasing voltage is coupled through a resistor to the rectifier diode. Variations of the rectifier diode threshold voltage due to temperature changes is compensated for because the voltage over the second diode changes in the same manner, if the diodes are at the same temperature and if they have equal temperature coefficients.